The present invention relates to a non-volatile memory device and a method thereof, and more particularly, to a phase changeable memory device and a method of formation thereof.
Non-volatile memory devices share a common characteristic in that they retain stored data even though the power supply is interrupted. Contemporary non-volatile memory devices commonly employ flash memory cells having a stacked gate structure. The stacked gate structure includes a tunnel oxide layer, a floating gate, a gate interlayer dielectric layer and a control gate electrode, which are sequentially stacked. In order to improve reliability of the flash memory cells and programming efficiency, the characteristics of the tunnel oxide layer should be improved and the coupling ratio of the cell should be increased.
To improve upon the characteristics of conventional flash memory devices, new flash memory device architectures, such as phase changeable memory devices, have been suggested. The phase changeable memory devices can perform the functions of programming and reading by changing the phase of a material layer, such as a GST (Ge2Sb2Te5) layer, between a crystalline phase and a non-crystalline phase according to the temperature of the material layer.
The conventional phase changeable memory devices have a structure in which a lower electrode contact directly contacts a phase changeable layer. When a current generated by a transistor flows through a lower electrode contact having a very small area relative to the phase changeable layer, heat is generated at the interface between the phase changeable layer and the lower electrode contact, in turn causing a phase change to occur in the phase changeable layer, since the lower electrode contact functions as a heater. A phase of the phase changeable layer is changed at the interface due to the heat. If the phase is changed, the resistance of the phase changeable layer is likewise changed. It is possible to program the phase changeable memory device by changing the phase of the phase changeable memory device. However, in a conventional structure where the lower electrode contact directly contacts the phase changeable layer, a large amount of current is required at the time of programming. Since thermal conductivity of the phase changeable layer is high, any heat that is present at the interface between the lower electrode contact and the phase changeable layer is output through the phase changeable layer, thereby causing a loss of heat. To achieve a phase change, the temperature at the interface must reach a specific temperature. However, a great deal of current is required for supplementing the lost heat.
For reducing the amount of current required at the time of programming, a conventional phase changeable memory device has a structure whereby an insulating layer is formed on a lower electrode pattern contacting the lower electrode contact and is patterned to form a contact hole in the insulating layer. The contact hole is then filled with a phase changeable layer to contact a portion of the surface of an exposed upper portion of a lower electrode through the contact hole. In the structure, an insulating layer having a low thermal conductivity is located at a lower portion of, and at both sides of an interface where the phase changeable layer contacts the lower electrode pattern, thereby preventing or mitigating heat loss. However, in the above structure of forming a memory device, the etching gas can react with a lower electrode layer to form an undesirable oxide layer and a fluoric layer at the interface, thereby causing a resistor distribution failure. In order to remove the unwanted oxide layer and the fluoric layer, a cleaning process is performed using a radio frequency (RF) plasma. At this time, the insulating layer at inner sidewalls of the contact hole to be filled with a phase changeable layer is etched to increase the width of the contact hole and therefore, the beneficial effects of a reduction in heat or current are decreased.